Continuing advances in laser technology have greatly increased the number of industrial and scientific applications for which lasers have been employed. Conventional laser systems may be employed, for example, to perform micromachining operations such as trimming thin-film resistors or repairing defective integrated circuits. Target structures such as these often have minimum critical dimensions of between 5 and 75 microns. Such micromachining operations generally employ and often require laser systems having output position accuracy and shape tolerances of about 1.0 micron on the target structure.
Certain laser systems have been designed to provide limited, but useful amplitude control of laser output. For example, U.K. Patent No. GB 1,545,933 of LASAG AG describes a pulsed-current pumped laser system that generates high amplitude laser output to provide "clean" incremental ablation to produce a hole of a desired depth in a target substrate by maintaining vaporization of molten substrate material during hole formation. The laser system employs a dedicated, nonversatile driving circuit, a resonator having an operation-specific length, and a Q-switch to provide limited, preselected, amplitude control of laser output. The system also employs a light switch to rapidly cut off the trailing edges of the output generated by the selectively long laser resonator.
U.S. Pat. No. 3,962,558 of Kocher et al. describes a pulsed-current pumped laser system that employs a succession of high amplitude laser output to create holes in watch jewels. The system increases the amplitude of the initial spikes of the output to create an absorbing state in transparent or reflective materials.
These laser systems are designed and constructed, however, to produce laser output having very narrow non-variable ranges of amplitude and duration for highly specific operations. Furthermore, the tolerances required by these applications necessitate meticulous and dedicated output-specific alignment of all the laser system optical components.
Increased laser utility has, however, created a demand for more versatile laser systems to perform a variety of operations employing vastly different extremes in output amplitude on target structures having even smaller critical dimensions. For example, U.S. Pat. No. 4,930,901 of Johnson et al. preferably employs a continuously current pumped laser system that, in conjunction with a Q-switch, generates high amplitude laser output suitable for severing electrical leads from a lead frame. The system is preferably driven by a programmable, high speed switching power supply capable of changing the current at a rate of 0.125 amp/.mu.sec. The switch timing of the power supply and the Q-switch is coordinated to provide output amplitude shaping control of both the leading and the trailing edges, as well as control of output duration and amplitude level. The versatile output amplitude shaping control permits the same continuously pumped laser system to simulate low amplitude pulse-current pumped laser output of selectable amplitude shape and duration for bonding leads to a printed circuit. Thus, the laser system can perform both bonding and severing processes.
One disadvantage of such a laser system is that whenever the laser system pumping energy or pumping pulse length is altered, the thermal loading of the laser rod is affected. The focal length of the laser rod shortens or lengthens whenever a higher or lower pumping energy is generated, respectively, or whenever the pulse length is increased or decreased, respectively. Such changes in the focal length adversely affect the spatial mode content, waist position, and divergence of the laser output.
A second disadvantage is that high speed changes in pumping extremes will wear out a typically expensive light source of such a laser system in a relatively short period of time. In addition, the pulse repetition rate of such a laser system is limited by the rate at which the current can be ramped between the below threshold level and the pumping level required to generate the desired output amplitude level.